The wipers in many vehicles wipe back and forth simultaneously in the same direction, generally referred to as a tandem system. Some windshields, however, are shaped and sized such that the wipers must wipe simultaneously and in opposite directions, from a lowermost, inner wipe position to an uppermost, outer wipe position and back. This is necessary in order to wipe a sufficient portion of the windshield surface, sometimes called the legal area. Sometimes, the windshield is so large that the wipers must be so long that they overlap at inner wipe, with one wiper resting on top of the other at inner wipe. In that case, the wiper drive mechanism must have a provision for preventing the top and bottom wipers from colliding both as they leave inner wipe at the beginning of a wipe cycle, and as they return to inner wipe at the end of the cycle. The typical production drive mechanism for such a system uses a bell crank linkage designed to assure that the top wiper will begin to move up from inner wipe faster than the bottom wiper at the beginning of the wipe cycle, and so prevent collision. However, an undesirable result of the bell crank type mechanism is that one of the wipers will experience sudden acceleration of "whip" at outer wipe, which can potentially take it past the ideal outer wipe position. Another drawback of the bell crank type drive mechanism is that it uses a number of large and heavy links, and occupies a large under hood volume.